Changing examples to use makefile

- Testing a modified version of the makefile from raylib Instead of multiple project files for examples.
- Fixed readme example

Examples/models/resources/shaders/skybox.fs

@@ -0,0 +1,31 @@
+/*******************************************************************************************
+*
+*   rPBR [shader] - Background skybox fragment shader
+*
+*   Copyright (c) 2017 Victor Fisac
+*
+**********************************************************************************************/
+
+#version 330
+
+// Input vertex attributes (from vertex shader)
+in vec3 fragPos;
+
+// Input uniform values
+uniform samplerCube environmentMap;
+
+// Output fragment color
+out vec4 finalColor;
+
+void main()
+{
+    // Fetch color from texture map
+    vec3 color = texture(environmentMap, fragPos).rgb;
+
+    // Apply gamma correction
+    color = color/(color + vec3(1.0));
+    color = pow(color, vec3(1.0/2.2));
+
+    // Calculate final fragment color
+    finalColor = vec4(color, 1.0);
+}